Field of the Invention
This invention relates to an electrolysis cell.
Discussion of Related Art
Electrolysis cells of this kind are generally known and are disclosed, for example, by German Patent References DE 10 2009 004 031 A1 and DE 34 01 637 A1. Through electrolysis of water, oxidizing agents can be produced at suitably embodied anodes and can be used for oxidative treatment of the water or for disinfection of the water. The processes that occur at the electrodes during electrolysis of natural water, e.g. tap water, are predominately the oxidation and reduction of water:At the anode: H2O→½O2+2H++2e−  (1)At the cathode: 2H2O+2e−→H2+2OH−  (2)
In the water, the current is transported by the dissolved ions. When a flow of current occurs, a pH gradient builds up in the electrolysis cell (pH <7 at the anode, pH >7 at the cathode). The pH gradient causes alkaline earth carbonate and/or alkaline earth hydroxide to precipitate on the alkaline cathode (“calcification”). Because of the low ion concentrations and the resulting low conductivity, the electrolysis of natural water is limited in the practicable current densities.
Through the use of a cation exchange membrane as an electrolyte (proton exchange membrane=PEM), it is possible to increase by several orders of magnitude the practicable current density of an electrolysis cell operated in water and to produce oxygen in the form of O2 and/or O3 at the anode. The membrane in this case is contacted over its entire area on both sides by the porous electrodes (anode and cathode). The processes occurring at high current densities at the electrodes in this arrangement are comprised of:At the anode: H2O→⅓O3+2H++2e−At the cathode: 2H++2e−→H2  (3)
In the cation exchange membrane, the current is transported by protons (H+) in accordance with equations 1 and 3 and is not limited by the ion concentration in the water. Primarily, the conduction of current with protons, which are present in a high concentration in the membrane, produces no pH gradients. But since the membrane is in chemical equilibrium with the water, cations dissolved in the water migrate into the membrane through ion exchange with protons and accumulate there. Even if the portion of the current transported by dissolved cations in a membrane cell is far less than one percent, it can influence the behavior of the cell significantly. Cations being exchanged at the anode generate a proton surplus (=acidification), as occurs in the cell without a membrane. Thermodynamically, the discharging of protons of hydrogen is the preferred reaction at the cathode. The metal cations that have migrated into the membrane therefore accumulate in the vicinity of the cathode and after a threshold concentration in the membrane at the cathode is reached, can lead to the production of hydroxides according to reaction (2). The operation of such known cells is limited by the presence of hardness components dissolved in the water, such as Ca2+ and Mg2+ ions in the form of dissolved hydrogen carbonates. The hardness components precipitate onto the cathode in the form of carbonate and/or hydroxide and therefore result in coatings that increase ohmic resistance of the electrolysis cell and limit the economically practicable operating times of such cells in natural water to a few hours.
Various methods are known from the prior art that make it possible to operate such electrolysis cells in hard water (i.e. with a large quantity of dissolved hardness components) in an economical way. PCT Publication WO 2012/159 206 A1 discloses dissolving cathodically generated deposits by reversing the polarity of the cell. The electrodes used for this, however, are very expensive to manufacture and the method requires disadvantageously large electrode surfaces. The patent specification of European Patent Reference EP 1 649 080 B1 discloses a cell in which a pre-electrode made of wire mesh is situated between the cathode and the membrane. This cell, however, also disadvantageously leads to a cell voltage of greater than 20 V after 100 hundred hours of operation. Alternatively, it is known to use conventional softeners to soften a partial flow of the water that is to be treated, but this is very complex in terms of both operation and maintenance.
The known approaches to solving the problem are comparatively complex and rather inefficient. One object of this invention, therefore, is to provide an electrolysis cell that does not have the disadvantages of the prior art and in which it is possible to prevent the formation of a barrier layer on the cathode so that the electrolysis cell can be operated even with hard water, i.e. in the presence of large quantities of hardness components, and can therefore be reliably used in natural water for longer periods of time.